This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for separating copy sheets having a toner powder image transferred thereto from a moving photoconductive surface.
In the process of electrophotographic printing, a charged photoconductive member is exposed to a light image of an original document being reproduced. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the informational areas contained within the original document. A development system moves a developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image thereon. Thereafter, the toner powder image is transferred to a sheet of support material. After transferring the toner powder image to the sheet of support material, a fusing device permanently affixes the toner powder image thereto.
During the transfer process, the sheet of support material is generally placed in contact with the toner powder image on the photoconductive surface and the backside of the sheet subjected to a spray of ionized air. This results in a charge being formed on the sheet having a magnitude and polarity sufficient to electrostatically attract the toner particles thereto. However, during transfer, a charge opposite to the charge found in the non-image areas of the photoconductive surface is induced on the sheet of support material. This causes the sheet to become electrostatically tacked to the photoconductive surface. The removal of the sheet of support material from the photoconductive surface has long been a problem in electrophotographic printing. Mechanical and pneumatic stripping devices have been used for quite some time in the printing art with varying degrees of success. However, devices of this type frequently suffer from misalignment problems. When the stripping mechanism is misaligned, it fails to act upon the sheet of support material either at the proper time or at the proper place. The sheet of support material may then remain on the photoconductive surface, i.e. it is not stripped, or the toner powder image is marred. In the case of a mechanical stripper wherein the pick off fingers are interposed between the photoconductive surface and the sheet of support material, misalignment of the fingers may produce abrasion or scratches on the photoconductive surface necessitating replacement thereof. Any of these malfunctions can seriously impair the reliability of the printing machine involving a great deal of lost machine time and, in the extreme case, result in permanent damage to the machine components. To this end, devices have been developed for floating the stripper finger on a cushion of air. This requires that the stripping fingers be spaced at a substantially constant distance from the photoconductive surface. However, a rotating photoconductive drum frequently has run-out which causes variations in the spacing between the stripping fingers and the photoconductive surface. To overcome this problem, the stripping finger has to be mounted movably. Hereinbefore, pneumatic devices, such as bellows, controlled the movement of the stripping fingers. Systems of this type generally have high spring constants requiring a large volume control chamber. This may result in a slow system response time causing errors which may result in mis-stripping.
Accordingly, it is a primary object of the present invention to improve the sheet stripping apparatus employed in an electrophotographic printing machine.